1. Field of the Invention
The invention is generally related to semiconductor processing. More particularly, the invention is related to re-metallizing aluminum bond pads on a semiconductor integrated circuit so that the re-metallized bond pads are solder-wettable.
2. Related Art
Cost-effective integration and packaging are the keys to the successful commercialization of electronic and optoelectronic components in large-volume markets.
Wire-bonding is one well known technique for forming an electrical connection between two semiconductor integrated circuits (ICs). During wire-bonding, metal electrodes (called bond pads) on the respective semiconductor ICs are electrically connected together using very small diameter wires (called bond wires) that are typically made of gold. Example diameters of bond wires are approximately 20-25 um. Since the bond wires are very thin, the length of the bond wires should be kept relatively short in order to prevent parasitic reactances from affecting the circuit performance. Additionally, the bond wires are fragile and cannot be relied on to provide a mechanically rigid connection between the semiconductor ICs.
Another well known integration technique is called xe2x80x9cflip-chipxe2x80x9d bonding. In flip-chip bonding, two semiconductor ICs having matching bond pad patterns, are soldered together by re-flowing solder bumps that are attached to the bond pads of one of the ICs. During flip-chip bonding, one of the ICs is xe2x80x9cflippedxe2x80x9d upside down and aligned with the matching bond pad pattern on the second IC. After which, the solder bumps are re-flowed to perfect the electrical and mechanical connection between the matching bond pad patterns on the two semiconductor ICs.
Flip-chip bonding has multiple advantages over wire-bonding, assuming there are matching bond pad patterns on the semiconductor ICs. For example, flip-chip bonding takes advantage of self-aligning properties of solder, which can compensate for IC misalignment up to a few microns. Additionally, all of the solder connections are re-flowed simultaneously, instead of individually. Additionally, flip chip bonding provides a rigid mechanical connection between the two semiconductor ICs. Finally, flip-chip bonding eliminates the parasitics associated with the long bond-wires that are used in wire-bonding.
Despite the advantages of flip-chip bonding, most silicon CMOS chips that are produced by commercial foundries have bond pads that are made of sputtered aluminum alloys, such as Al/Si/Cu(98%, 1%, 1%). These aluminum alloys are not solder-wettable, and therefore cannot be flip-chip bonded without modification.
Therefore, what is needed is a process for re-metallizing aluminum alloy bond pads so that the bond pads are solder-wettable, and can be flip-chip bonded.
The present invention is directed to a re-metallized aluminum bond pad on a semiconductor integrated circuit (IC), and a method or process of making the same. The re-metallized bond pad includes the following layers: the original aluminum layer, a zinc layer, a nickel layer, and a gold layer. The re-metallized bond pad is both wire-bondable and solderable, and can be flip-chip bonded.
The re-metallization process is an electroless plating process that re-metallizes the aluminum pads with gold. The process works selectively on the aluminum pads only, without being detrimental to the underlying silicon circuitry that is typically protected by a passivation layer.
The first step of the re-metallization process is to pre-clean the aluminum bond pad to remove dust and wafer processing residue. The pre-cleaning can be done with organic solvents, including TCA, Acetone, and Methanol.
The next step of the re-metallization process is to de-oxidize the aluminum bond pad to remove the native oxide layer. This can be done by micro-etching the aluminum bond pad in an acid solution. The concentration and immersion time can be varied to adjust the roughness of the resulting de-oxidized aluminum surface.
The next step in the re-metallization process is to deposit a layer of zinc on the de-oxidized aluminum using an alkaline zincate solution. Preferably, the zinc layer is applied in two zincate treatments, called xe2x80x9cdouble zincation.xe2x80x9d A first seed layer is applied immediately after de-oxidizing the aluminum bond pad, to prevent the aluminum bond pad from re-oxidizing. The second zinc treatment is performed after xe2x80x9cdesmuttingxe2x80x9d the IC in nitric acid. Double zincation is preferred over single zincation, because the intermediate de-smutting step strips the granulated initial zinc deposit, and produces a more uniform zinc film over the surface of the bond pad. Additionally, superior uniformity in surface coverage and grain size has been achieved by raising the temperature of the zincate solution to a range of 38-42 degrees C. during immersion. In contrast, the manufacturer of the zincate solution suggests a temperature of 25 C, or room temperature.
The next step in the re-metallization process is to deposit a layer of nickel onto the zinc layer using an electroless process. The nickel layer seals the aluminum surface as a solder diffusion barrier layer, and also provides hardness, mechanical strength, and solderability to the bond pad.
The final step in the re-metallization process is to deposit a layer of gold onto the nickel layer using an immersion process and/or an autocatalytic process. The gold layer protects the re-metallized bond pad from oxidation, and improves solderability and wire-bondability.
The re-metallizing process is a low-cost and efficient technique for re-metallizing aluminum bond pads. The process is low cost because the electroless plating solutions that are utilized are readily available in the commercial market. Additionally, no electric current source is necessary because all the solutions and steps are electroless. Additionally, the number of steps in the process cycle have been minimized by selecting advantageous combinations of chemical reagents. Potentially hazardous steps, like cyanide zinc pre-treatment and cyanide autocatalytic gold plating are replaced with benign alternative processes.
Another advantage of the invention is that the re-metallized aluminum/gold bond pads are still wire-bondable. In other words, the re-metallized bond pads are suitable for both flip-chip solder bonding, and manual wire-bonding applications. This results in cost and time savings during hybrid assembly. Additionally, no additional masking or lithographic processing is utilized to protect the bond pads in order to insure wire-bondability.